Asters provide a burst of color to the late-season garden, often persisting long after other blooms have faded. As perennials, they return each year, leading many gardeners to wonder about their potential for expansion. Asters do spread, but the degree of expansion is highly variable and depends on the specific type planted. Understanding how these plants propagate is key to managing their presence within a landscape.
Mechanisms of Aster Spread
Asters utilize two main strategies to colonize new areas. The most aggressive form of expansion is vegetative, occurring beneath the soil surface through specialized underground stems called rhizomes. These horizontal runners grow outward from the parent plant, periodically sending up new shoots that develop into genetically identical plants. This process forms dense, ever-widening patches that can quickly crowd out neighboring vegetation.
The second method of propagation is self-seeding, which involves the dispersal of seeds. After the flowers fade in the fall, they produce numerous small, feathery seeds, similar to those of a dandelion. These seeds are easily carried by the wind, sprouting new aster plants in unexpected locations the following spring.
Differences Among Common Aster Varieties
The growth habit of an aster is linked to its species, with some types being far more assertive than others. Asters classified as “aggressive runners” rely heavily on rhizomatous expansion and require frequent intervention. The New England Aster (Symphyotrichum novae-angliae) and the New York Aster (Symphyotrichum novi-belgii) are examples that spread via a network of rhizomes. These species form large clumps that expand outward noticeably each season, demanding regular division to maintain their size and vigor.
Other varieties, referred to as “clump-formers,” exhibit a more restrained growth pattern. Species like the Smooth Aster (Symphyotrichum laeve) and the Aromatic Aster (Symphyotrichum oblongifolium) expand slowly from the center, maintaining a compact root ball. Their rhizomes are shorter and less vigorous, meaning they take much longer to encroach on surrounding plants.
Controlling Unwanted Aster Expansion
Managing the spread of asters requires targeting both underground runners and airborne seeds. For aggressive varieties, such as the New England or White Wood Aster (Eurybia divaricata), planting them within a physical barrier prevents rhizomatous spread. This is achieved by sinking a root barrier or a bottomless container into the soil before planting, corralling the lateral growth of the rhizomes. Alternatively, selecting a location with natural boundaries, like a paved walkway or a foundation, limits expansion.
To combat vegetative spread, annual or biennial division is necessary. This involves digging up the entire clump and removing the vigorous outer sections. Replanting only a small, healthy section prevents overcrowding and maintains plant health.
Self-seeding is controlled by removing spent flower heads before the seeds fully mature and disperse. This practice, known as deadheading, should be performed in late fall after the blooms have finished their display.